SAT Question 0b332f00: Linear Functions answer and explanation with Desmos solution

It's given that $g\left(x\right)=6x$. Substituting $54$ for $g\left(x\right)$ in the given function yields $54=6x$. Dividing both sides of this equation by $6$ yields $x=9$. Therefore, the value of $x$ when $g\left(x\right)=54$ is $9$.